This invention relates to fiber-reinforced plastic moldings, and particularly to moldings and molding compositions which possess an especially advantageous combination of properties for certain applications, particularly a combination which includes both precision-molding properties and high abrasion resistance.
Thermosetting molding compositions containing mineral fillers and reinforced with mineral fibers are widely used to provide electrical and mechanical components having advantageous physical, chemical, and electrical characteristics.
For example, such compositions are employed to form intricate moldings used as the housings and frames of portable power tools and moldings used in assembly with each other and with contacts and other electrical elements to form electro-mechanical contactors and the like which include a movable contact carrier actuated by a built-in solenoid. In such applications, the moldings are subjected to both rubbing abrasion and impact abrasion from movement of the contact carrier and also to abrasion from electrical and mechanical vibration. For such applications, moldings made with mineral fillers such as aluminum hydrate, glass fibers, and with polyester or epoxy resins have many advantageous properties, for example, convenient and economic manufacture, good dimensional stability, high mechanical strength, good corrosion resistance, and good electrical characteristics, including high dielectric strength and high arc-resisting, arc-quenching, and non-tracking properties, but such moldings do not have adequate abrasion resistance.
It is known from Robert E. Wilkenson U.S. Pat. No. 3,397,169 that the abrasion resistance of such mineralfilled thermosetting molding compositions can be substantially improved by the addition to the mineral-filled thermosetting molding composition of a small proportion of polyethylene or other polyolefin which is solid at room temperature. In that U.S. Pat. No. 3,397,169, the abrasion-improving addition is made to general purpose thermosetting resin compositions, and the patent specifically mentioned polyester resins which contain free-radical initiators and modified epoxy resins such as epoxy-acrylate resins which are reactive in the presence of a free-radical catalyst.
The compositions of that prior Wilkinson patent have a relatively high degree of shrinkage during curing, with the result that they do not have precision-molding characteristics and moldings made therefrom are subject to warpage and do not have high precision and accuracy. In moldings which are to form the assembled parts of an electro-mechanical relay or switch, such as a contactor, and in many other applications, a degree of precision is required which often exceeds that obtainable by molding such compositions, and use of such compositions in these applications has required special steps to overcome warpage and secure accuracy, such as machining to meet critical dimensions.
Improved precision-molding characteristics can be obtained with certain mineral-filled thermosetting molding compositions by using in the composition a low-profile resin system which includes a "low-shrink" component. Low-profile resin systems and molding compositions are shown, for example, by the following patents:
__________________________________________________________________________ British No. 936,351 Publ. 9/11/68 F. J. Parker et al. British Industrial Plastics Ltd. U.S. Pat. No. 3,549,586 12/22/70 P. L. Smith et al. Union Carbide Corp. U.S. Pat. No. 3,668,178 06/06/72 L. R. Comstock et al. Union Carbide Corp. U.S. Pat. No. 3,701,748 10/31/72 C. H. Kroekel Rohm & Haas Company U.S. Pat. No. 3,718,714 02/27/73 L. R. Comstock et al. Union Carbide Corp. __________________________________________________________________________
In general, these use a low-profile resin system which includes a thermosetting unsaturated polymer such as a polyester and a monomer such as styrene, and includes as a low-shrink additive therein a thermoplastic polymer which is soluble in the monomer but is not converted to a thermosetting material by or in the presence of the polymerization of the resin system.
These low-profile compositions were developed primarily to improve the surface finish of moldings which were to be painted and on which an especially smooth surface was needed. The same low-shrink characteristics which give good surface finish also give precision molding characteristics which are highly desirable in forming moldings which require high mechanical accuracy. It is found, however, that the low-profile molding compositions have very poor abrasion resistance, and while the teachings of the Wilkinson U.S. Pat. No. 3,397,169 may be used to improve the poor abrasion resistance of low-profile compositions, the improvement is not sufficient to overcome the poor abrasion resistance, and the resulting moldings do not have adequate abrasion resistance for the applications here contemplated.
This failure was especially critical in the development of an improved electro-mechanical contactor switch in which both precision molding and high abrasion resistance were essential, along with other properties provided by mineral-filled thermosetting polyester molding compositions. There are numerous other applications where this combination of properties would be of high value.
By precision molding properties, we mean that the composition may be molded in precision dies and will form molded parts in which little or no shrink occurs during cure and which match the dies with a high degree of precision, for example, with a variance therefrom of not to exceed .+-.0.0005 inch per inch and with little or no warpage or distortion. This compares with a variance of .+-.0.003 inch per inch or more and warpage problems with corresponding moldings made with general purpose molding composition of a comparable formula not including a low-profile resin system. This is an improvement by a factor of 6 in an accuracy range which is already highly exact.
The very poor abrasion resistance of low-profile molding compositions is not well understood, but it evidently involves the resin system and its characteristics and behavior in relation to the fillers and reinforcing fibers and other components of the composition.